Nickel-copper Bessemer matte is typically produced by converting molten matte from a primary smelting furnace in Peirce Smith converters which employ blowing of air or air/oxygen mixtures into the bath via tuyeres. The Peirce Smith converter is the most common type of converter used for this application and consists of a horizontally oriented cylinder which has a hooded opening at the top and is rotatable through an arc of about 180 degrees. The plurality of tuyeres are located below the normal working level of the molten matte when in the blowing position. As a result of converter rotation, the tuyeres are above the bath for pouring and holding.
The objective of the conversion process is to oxidize the FeS in the matte to form iron oxides, liberating sulfur dioxide and leaving matte comprising nickel and copper sulfides with small but variable amounts of cobalt, precious metals and dissolved oxygen. This is accomplished by blowing an oxygen containing gas (air or oxygen enriched air) into the matte through the tuyeres. The oxygen combines with the iron and sulfur to form iron oxide and sulfur dioxide. The sulfur dioxide passes off as a gas and is subsequently treated to prevent fugitive emissions. The iron oxide unites with added silica flux to form an iron silicate slag that floats on top of the matte now richer in nickel and copper and much lower in iron. The oxidation process is exothermic and the heat generated is usually sufficient to cause the operation to be self-sustaining. Additions of fuel are typically not required.
After removal of substantially all of the iron by blowing and skimming of the slag, the resulting matte is cooled, cast and further treated for recovery of the valuable base and precious metals. Upon cooling, the copper and nickel in the matte form copper sulfide (Cu.sub.2 S), nickel sulfide (Ni.sub.3 S.sub.2), and a metallic fraction containing small amounts of dissolved sulfur.
Turning to the tuyeres, compressed (or blast) air is delivered through a header disposed along the back of the converter. The header, generally delivering the blast air at about 15 pounds per square inch (103 kPa), feeds each tuyere. A plurality of horizontal tuyeres provide direct air passages through the converter lining into the interior of the converter.
After the converter is filled to the appropriate working level with the desired material, the tuyeres are above the level of the charge. The blast air supply is turned on and the converter is rotated to submerge the tuyeres a predetermined distance below the surface of the charge. As the tuyere air bubbles up through the charge, the desired oxidation processes occur.
Over time solid accretions begin to accumulate within each tuyere ultimately causing it to plug up. In order to keep the tuyeres open, a reciprocating rod is inserted into the tuyere. The rod is connected to a pneumatic valve that causes the rod to traverse the tuyere and literally punch out the accumulated mass back into the converter. An automatic pneumatic tuyere punch including the rod and the valve body is mounted to the exterior of the converter over the tuyere. At regular intervals the valve is energized to first ram the rod into the tuyere and then retract it. By repeating this process, the tuyere remains open to allow blast air to enter the converter.
With the increasing need to clean up industrial processes, reduce waste and pollution, and increase efficiency and recovery rates, it has been proposed to recycle certain materials back into the converter when possible for additional processing. Copper and nickel concentrates, electrostatic precipitator dust, catalytic converter dust and other materials may be fed into the converter for good effect.
Unfortunately, everything added to the charge must be currently done on a batch basis. Materials cannot be introduced into the converter in a steady, continuous stream. Moreover, by dumping material into the active converter, the material acts as a chill adversely affecting the temperature of the bath. Also, due to the blast effect, it is difficult to evenly introduce light weight materials, such as dust, into the converter without having them being blown out.
An example of an attempt to introduce fuel into a converter is disclosed in U.S. Pat. No. 4,711,433. A blowing pipe assembly is designed to be removed before the charge is introduced into the converter. After the converter is rotated into the upright/blowing position, the blowing pipe assembly is then physically remounted to the tuyere. Besides requiring a constant repeated mounting and dismounting operation, there is no apparent acknowledgement of the problems concerning a plugged tuyere.
Another example of an attempt to introduce gas into an argon-oxygen-decarburization (AOD) vessel is shown by U.S. Pat. No. 4,795,138. A tuyere having inner and outer concentric tubes permits oxygen to flow within the central tube and an inert gas to flow within the outer tube so as to control the flow ratio of the gases entering the vessel.
Accordingly, there is a need for a technique that permits the expeditious, continuous introduction of particulate material into a converter and, more particularly, directly into the bath.